1. Field of the Invention
The present invention relates to a positive resist composition and a method of forming a resist pattern using the same, the composition being suitable for use in microlithography processes such as the production of ULSIs or high-capacity microchips and in other photofabrication processes. More particularly, the invention relates to a positive resist composition capable of forming a high-resolution pattern with a vacuum ultraviolet light having a wavelength of 160 nm or shorter.
2. Description of the Related Art
The degree of integration in integrated circuits is increasing more and more, and it has become necessary to form an ultrafine pattern having a line width of a quarter micrometer or smaller in the production of semiconductor substrates for ULSIs or the like. One of the known techniques for enhancing the fineness of such patterns is to use an exposure light having a shorter wavelength in resist pattern formation.
For example, in the production of semiconductor elements having an integration degree of up to 64 megabits, the i-line (365 nm) from a high-pressure mercury lamp has hitherto been used as an exposure light. Many compositions comprising a novolak resin and a naphthoquinonediazide compound as a photosensitive substance have been developed as positive resists for use with the exposure light, and have given sufficient results in forming patterns having line widths down to about 0.3 μm. On the other hand, in the production of semiconductor elements having an integration degree of 256 megabits or higher KrF excimer laser light (248 nm) has been used as an exposure light in place of the i-line.
Furthermore, use of ArF excimer laser light (193 nm), which is an exposure light having an even shorter wavelength, and use of F2 excimer laser light (157 nm) for forming a pattern having a line width of 0.1 μm or smaller are recently being investigated for the purpose of producing semiconductor elements having an integration degree of 1 gigabit or higher.
With the use of such exposure lights having shorter wavelengths, the components of resist materials and the structures of compounds for use therein are changing considerably. Since the resists heretofore in use which comprise a novolak resin and a naphthoquinonediazide compound show considerable absorption in a 248 nm far-ultraviolet region, the light is less apt to reach the resist bottom. Namely, such resists have low sensitivity and the patterns obtained therefrom have been limited to ones having a tapered shape.
For eliminating such problem, a so-called chemical amplification type resist has been developed. This resist composition comprises a combination of: a resin, as the main component, having a poly(hydroxystyrene) backbone which shows reduced absorption in a 248 nm region and is protected by acid-decomposable groups; and a compound which generates an acid upon irradiation with far ultraviolet light (photo-acid generator); In the chemical amplification type resist; an acid generates in exposed areas and this acid catalyzes a decomposition reaction to thereby change solubility in a developing solution. Because of this, the resist can have high sensitivity and form a high-resolution pattern.
However, in the case where ArF excimer laser light (193 nm) is used, even the chemical amplification type resist does not show sufficient performances because the compound having aromatic groups intrinsically shows considerable absorption in a 193 nm wavelength region.
An improvement in chemical amplification type resists is being attempted for overcoming the problem described above. This improvement is based on the use of an acid-decomposable resin which has, incorporated in the polymer main chain or side chains thereof, alicyclic structures showing no absorption at 193 nm, in place of the acid-decomposable resin having a poly(hydroxystyrene) backbone.
However, with respect to use with F2 excimer laser light (157 nm), even the alicyclic resin was found to be insufficient in obtaining a desired pattern of 0.1 μm or finer because the resin shows considerable absorption in a 157 nm region. Under these circumstances, a resin having fluorine atoms (perfluorinated structure) incorporated therein was reported to have sufficient transparency at 157 nm, in, e.g., Proceedings of the Society of Photo-Optical Instrumentation Engineers (Proc. SPIE), Vol. 3678, p. 13 (1999). Effective structures of such fluororesins are proposed in Proceedings of the Society of Photo-Optical Instrumentation Engineers (Proc. SPIE), Vol. 3999, p. 330 (2000), Proceedings of the Society of Photo-Optical Instrumentation Engineers (Proc. SPIE), Vol. 3999, p. 357 (2000), Proceedings of the Society of Photo-Optical Instrumentation Engineers (Proc. SPIE), Vol. 3999, p. 365 (2000), and WO-00/17712.
The resists containing such fluororesins, however, have peculiar water and oil repellency attributable to the perfluorinated structure. There has hence been a desire for an improvement in applicability (coating surface evenness) and diminution of development defects.
JP-A-2001-66779 discloses a resist composition containing a phenolic resin. In this technique, an improvement in the diminution of development defects and line edge roughness is attempted by incorporating a sulfonium salt having a hydroxy group.
Furthermore, there has been the following problem. Since triphenylsulfonium triflate and triphenylsulfonium nonafluorobutanesulfonate, which have hitherto been used as acid generators, do not dissolve in developing solutions, use thereof in combination with a water-repellent fluororesin makes the resist surface poorly soluble and this tends to result in reduced sensitivity and development defects.